Nowadays cyanide leaching is considered as the best option for leaching gold from gold-containing ores and concentrates. Another option is chloride leaching, but even though the use of chloride for gold leaching has been known for a long time, it has not been extensively applied in commercial practice. Chloride leaching has been considered too expensive due to relatively high reagent (oxidant) consumptions. The recovery of gold from chloride leach solutions has also been considered challenging, and the recovery yield has been lower than in cyanide leaching. Furthermore, there are corrosion problems because of high chloride concentrations of the leaching solutions.
U.S. Pat. No. 4,551,213 discloses a method of recovering gold values from sulphur-containing ore mixtures by leaching with chlorides (alkali metal chloride and/or alkali earth metal chloride), oxidizing agent (chloride gas) and cupric or ferric chloride. It is recited that the concentration of chloride in the leaching may be 12 to 47 weight-% and the concentration of cupric or ferric chloride 3.5 to 27 weight-%. A leaching temperature of about 20 to 106° C., preferably about 40 to 80° C., is disclosed. In the examples, the leaching is performed at a temperature of 60 to 65° C. The sulphur-containing ore used as the starting material may be obtained from hydrometallurgical processing of copper, for example. Gold may be recovered from the leach solution electrolytically or by adsorption on carbon.
EP 1 583 848 B1 discloses a process for recovering precious metals, such as gold, from sulfidic materials, by leaching with a mixture containing a mixture of halides and multivalent metal compounds selected from copper, iron, cobalt, manganese and vanadium compounds. Typical starting materials for the process are refractory gold concentrates, such as arsenopyrite (optionally with pyrite and/or with carbon). The mixture of halides in the leaching preferably comprises chloride and bromide. The multivalent metal in the metal halide leaching solution is typically copper (Cu2+). The leaching may be performed at a temperature of 90-105° C. Leaching pH values in the range of less than 1 but greater 0.2 are disclosed. The precious metal may be recovered from the each solution by adsorption on carbon (activated carbon), or by on exchange, solvent extraction, etc. From the recovery stage, the solution is typically recycled to the leaching stage.
According to Examples 1-3 of EP 1 583 848 81, gold was recovered from a single refractory Au concentrate by a two-step or one-step leaching process at a temperature of 105° C. with a leach solution comprising NaCl, CaCl2, Cu (from CuCl2) and HCl.
CA 2 599 174 discloses a method of leaching gold from copper sulphide ores, where the sulphide ore is first subjected to leaching of copper, followed by leaching gold from the leach residue with a leach liquor containing chloride ions and ferric and ferrous ions. It is also recited that the rate of gold leaching reaction can be enhanced by the co-presence of either copper or bromide on or both together in the leach liquor. A leaching temperature of 80° C. or more is disclosed. According to Example 6, copper is first leached from a copper sulphide concentrate, whereafter gold was extracted from the leach residue with a leach liquor containing Cu, Cl and Br at a temperature of 85° C.
Further processes for gold recovery have been disclosed in the textbook “The Chemistry of Gold Extraction”, J. Marsden and lain House. Society for Mining. Metallurgy and Exploration Inc., 2006, p. 275. These processes include a commercial leaching process of a silver-bearing material at 75° C. with a near-saturated solution of sodium chloride with hydrochloric acid (pH 0.3) and 15 g/l ferric ion as the oxidant. A further process comprises an atmospheric leaching process using NaCl and NaOCl at pH 7 from gold ores containing cyanide-soluble copper. A still further process consists of high temperature (200 to 225° C.) pressure oxidation in sulphate media containing 5 to 20 g/l NaCl, particularly proposed for gold-bearing starting materials containing by-products such as copper, nickel, and platinum group metals.
WO 2011/154603 A1 (15 Dec. 2011) discloses a method of recovering gold by solvent extraction from an acidic chloride-containing aqueous solution or from a slurry containing gold-bearing solids. The solvent extraction is performed with an extractant containing a diester of 2,2,4-trialkyl-1,3-pentanediol. The gold-bearing organic solution obtained from the extraction is scrubbed with an acidic aqueous solution, after which gold is stripped into water, from where it is reduced to form pure gold.